Substrat- und Adsorbatphononen der reinen und der wasserstoffbedeckten W(110)-Oberfläche

Balden, M.

Book/Report

FZJ-2019-01053

Substrat- und Adsorbatphononen der reinen und der wasserstoffbedeckten W(110)-Oberfläche

Balden, M. (Corresponding author)FZJ*

1995
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Berichte des Forschungszentrums Jülich 3148, 114 p. (1995)

Please use a persistent id in citations: http://hdl.handle.net/2128/21511

Report No.: Juel-3148

Abstract: The adsorbate and substrate surface phonons of the hydrogen and deuterium covered W(110) surface were investigated using high resolution electron energy lossspectroscopy (EELS). The substrate surface phonon dispersion curves, especially of the Rayleigh wave (RW), were measured along the $\overline{\Gamma} \overline{H}$ $\overline{\Gamma} \overline{N}$, and $\overline{\Gamma} \overline{S}$ direction for the clean surface, the ordered p(2 x 1) and (2 x 2) phase, and for the (1 x 1) phase of hydrogen on W(110). The dispersion curves agree with the data obtained by He-scattering where such data is available. For the clean surface, at the $\overline{\Gamma}$-point, a dipole active surface resonance at about 190 cm$^{-1}$ has been resolved and confirmed by slab calculations using central forces up to third neighbors. For coverages below saturation the measured substrate phonon dispersion curves show a continuous change of the RW frequency around the $\overline{S}$ and $\overline{N}$ points with increasing coverage. Up to the coverage of the (2 x 2) phase, the frequency of the RW goes down at the $\overline{S}$ point by -18% and upwards at the $\overline{N}$ point by +14% compared to the clean surface. No effect of the suggested top layer shift reconstruction could be observed. For the saturated (1 x 1) phase the frequency of the RW, which forms the shallow dip of the previously observed surfacephonon anomaly at 0.93 $\mathring{A}^{-1}$ along $\overline{\Gamma} \overline{H}$, depends on coverage and temperature. In addition a shallow dip also for the longitudinal tungsten surface phonon at the same incommensurate wave vector along $\overline{\Gamma} \overline{H}$ was found. The frequency stays always above the RW. The deep dip previously observed with He-scattering, which has recently been interpreted as due to electron-hole pair excitations, has not been detected up to now with EELS at all accessible scattering conditions. At the wave vector of the anomalies along $\overline{\Gamma} \overline{H}$ we observed a small peak in the elastic diffuse intensity. Concerning the hydrogen vibrations, for the ordered p(2 x 1) phase three H-modes are resolved (550, 770, 1250 cm$^{-1}$). At least two of them are dipole active. The ordered (2 x 2) phase shows six sharp H-modes, suggesting that the two kinds of H-atoms in the (2 x 2) unit cell produce each a triplet of losses with frequencies comparable to the three p(2 x 1)-modes. The line width of the modes depends on the order of the overlayer, the coverage, and the adsorbate isotope. The (triply coordinated) hollow site could be determined as the adsorption site starting from low coverages up to above 0.75 ML. The H-modes of the (1 x 1) phase show an anomalous behaviour: they create in the in-specular EELS spectra a loss continuum up to 850 cm$^{-1}$ and a single sharp loss at 1300 cm$^{-1}$. Apossible explanation is the H-atoms being in atwo dimensional quasi liquid like phase. Additional structures in the frequency range of the continuum develop with dncreasing coverage and decreasing temperature. The origin of the anomalous behaviour of the hydrogen modes and a possible convection with the anomaly in the substrate surface phonons is still in discussion.

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